1. Field of the Invention
This invention relates to a process for the recovery of metallic gold from gold-containing thiourea, thiosulfate or thiocyanate solutions. The invention further relates to a process for the recovery of metallic gold generally of high purity from pregnant solutions as a result of the leaching of gold ore or concentrates by acidified thiourea.
2. DESCRIPTION OF THE RELATED ART
Hitherto, there have existed two principal methods of recovering gold from gold ores or gold concentrates. The first method involves cyanidation followed by the Merill-Crowe process wherein gold is recovered from solution by cementation with zinc powder which must then be refined to obtain gold metal. The process offers high gold recovery, but with low purity. The second method comprises cyanidation followed by recovery using activated carbon followed by electrolysis. The carbon-in-pulp (CIP) process involves contact between the activated carbon and leached pulp. Absorption of impurities onto the carbon and the difficulties of desorption of the gold are distinct disadvantages of this process. The carbon-in-leach (CIL) process involves loading the gold onto the carbon during leaching. In both the CIL and CIP processes, the precious metal must be eluted and passed to an electrowinning step for gold recovery. Gold recovered on the cathode then requires further refining. The activated carbon can be regenerated and then recycled.
Each of the these prior methods involves cyanidation. Because of the toxicity of cyanide, additional steps are required for its handling and subsequent elimination. This significantly increases the operating costs of these processes.
A recently proposed less toxic alternative to cyanide for leaching gold ore or gold concentrate is thiourea. However, thus far, there is no well established method of recovering gold from non-toxic reagents such as thiourea solution. Attempts have been made to recover gold by thiourea leaching followed by precious metal recovery from solution by aluminum cementation, activated carbon, ion exchange, solvent extraction and electrolysis. However, these prior methods all have one or more disadvantages. As well, there is often thiourea decomposition during gold recovery from the pregnant solution which adds cost by decreasing the amount of barren thiourea that can be recycled.
Thiourea is currently an effective eluate for resin loaded with gold cyanide complex and as a stripping agent for gold from organic solvent loaded with gold from cyanide media. However, no satisfactory method has been developed to recover gold from such solutions.
One method of recovering gold from such thiourea solutions involves neutralizing the acidified thiourea solution to a pH of about 6.5 which results in the precipitation of gold due to pH change. However, the method is non-selective and uneconomical due to the acid consumption necessary to readjust the solution pH if the thiourea is to be recycled. As well, thiourea is relatively unstable at a pH above 4.
Hydrogen reduction is another method that has been used to recover gold from thiourea solution. This process requires high temperatures and pressures and the use of a catalyst which contributes to high operating costs. As well, the reaction kinetics are quite slow.
Electrolysis has also been used in the recovery of gold from thiourea solution on a commercial basis. This method requires an elaborate two-stage electrolysis circuit with special cell design in order to obtain sufficiently high recoveries.
The reducing power of sodium borohydride has long been exploited for industrial applications such as pollution control and the removal and/or recovery of various metal cations from solution. Currently, sodium borohydride is finding application in the recovery of silver from spent photographic liquor (thiosulfate solution), as disclosed in U.S. Pat. No. 3,082,079, or spent electrolyte and platinum group metals from acidic leach liquor. Also, heavy metal cations such as Cu.sup.2+, Fe.sup.3+/2+, Ni.sup.2+, Hg.sup.2+, Co.sup.2+ and Pb.sup.2+ can be removed from toxic effluents by sodium borohydride treatment. However, there has been no suggestion of a sodium borohydride reduction process for the recovery of gold from leach liquors. Dietz, Jr. et al (Canadian Patent No. 1,090,584) teach a reduction precipitating agent containing aluminum, an alkali metal borohydride and a hydrazine compound for recovering precious metal values including gold from aqueous alkaline cyanide solutions. This prior process suffers from cyanide effluent problems as well as material losses due to the necessity for cyanide effluent destruction.